Electrolytic silver recovery from used photographic fixers is a common way to extend the lifetime of these fixers.
Important in the silver recovery process is the control of the electrochemical processes taking place at the anode and the cathode. There are a number of ways to operate an electrolytic desilvering cell. In many setups, a constant anode versus cathode potential is applied. When the desilvering of the solution reaches its end, a decrease in the electrolytic current occurs, and in this type of cells the process is usually shut off when the current decreases below a determined preset value. The disadvantage of the approach is that the deposition potential is not exactly controlled in many practical situations, and that the actual potential difference between the cathode and the solution (the "cathode potential") is unknown and varies during desilvering, causing unneccessary side reactions or a not necessarily optimal desilvering speed. In other setups, galvanostatic desilvering (constant current) of the fixer solution is carried out. In this setup, it is important to shut off the current when the silver content drops below a certain value, since unwanted side reactions and eventually sulphiding of the electrode may occur. Using more intelligent electronic circuitry, it is conceivable to develop setups which control the electrolytic desilvering cell using the cell resistance and the dependence of the cell resistance on the applied anode-cathode potential difference (e.g. first and second derivative of the current versus potential curve).
The above setups all suffer from the disadvantage that the actual plating potential is often not known when used in practical applications where the actual fixing solution to be desilvered consists of the starting pure fixing solution and a number of other components such as developer carried over from the developer tank, replenishment solution, additives, reaction products of development or of a previous electrolytic desilvering, etc.
If the desilvered fixing solution is to be reused, it is desirable to minimize the side reactions taking place at the anode and cathode which would give rise to unwanted by-products.
Three electrode setups, as commonly used in electrochemical instrumentation such as polarography instruments, allow a much better control of the silver deposition conditions, since the potential difference between the cathode and the fixer solution can be controlled. In this setup, the potential difference between cathode and anode is controlled by a feedback mechanism which keeps the potential difference between the cathode and the reference electrode, used to monitor the potential of the solution, at a constant value (potenstiostatic control). This allows optimal control of the plating reaction, since the reactions taking place at the electrodes are essentially controlled by the potential difference between the electrode and the solution.
In order to achieve a low residual silver level in the desilvered solution, and high desilvering speeds, the cathode potential should be kept sufficiently low, meaning sufficiently negative, versus the reference electrode. The lower the potential of the cathode, however, the more unwanted side reactions, e.g. sulphite ion reduction, are likely to occur. At still lower (very negative) potentials, sulphiding (formation of Ag.sub.2 S) of the cathode occurs. These side reactions at the catode not only consume sulphite but are inevitably accompanied by side reactions at the anode giving rise to supplemental unwanted by-products. In order to avoid these side reactions, it is therefore desirable to work at the lowest potential of the cathode not giving rise to these side reactions.
In establishing the optimal cathode potential for desilvering some problems occur when conventional reference electrodes are used.
(1) Reference electrodes which are known to be used as reference electrodes for electrolytic desilvering instruments are e.g. calomel type electrodes or Ag/AgCl electrodes as disclosed in scientific publication "Three-electrode control procedures for electrolytic silver recovery", Austin C. Cooley, Journal of Imaging Technology, volume 10, Number 6, December 1984, pagina 233-238. In view of its ecological implications the calomel type electrode containing Hg is not a desirable option. On the other hand Ag/AgCl electrodes need maintenance, especially when used in a solution tending to dissolve the materials used in the reference electrode. Other possible reference electrodes usually need maintenance if they are to provide stable potentials on the long term. when continuously used in a fixer solution. Moreover many commercial reference electrodes are not pressure compensated and are therefore not the best solution for use in systems where the fixing solution is pressurized (hydrostatic pressure, or pressure e.g. generated by pumps, etc . . . )
(2) The potential at which the reduction of sulphite starts to take place is dependent on the pH of the fixing solution. Therefore, the potential of optimal desilvering is dependent on the nature of the fixer used and on other parameters such as the pH of the developer bath, the presence or the absence of intermediate rinsing, the degree of carry-over from developer to fixer (dependent itself on e.g. the film type), the buffering capacities of the developer and the fixer solution, etc. In practical terms this means there is no common potential of optimum desilvering for various fixers having different pH values. For optimal desilvering, every fixer solution with a different pH would require a different potential difference between the reference electrode and the cathode. Therefore adjustments are necessary when the pH of the fixing solution changes due to differences in pH as a result of e.g. the use of additives, differences in carry over, or pH variations due to the reaction products of development or to a previous electrolytic desilvering.
It is an object of the present invention to provide an apparatus for the electrolytic desilvering of used photographic fixers or bleach-fixers which allows the establishment of an optimal desilvering potential which is independent over a broad range of the pH of the fixer or bleach-fixer.
It is a further object of the present invention to provide an electrolytic desilvering apparatus comprising a reference electrode which requires little or no maintenance.
It is a still further object of the present invention to provide an electrolytic desilvering apparatus comprising a reference electrode which is insensitive to hydrostatic pressure variations.